1. Field of the Invention
The present invention relates to a charged particle beam exposure method and more specifically to a charged particle beam exposure method whereby the overall desired pattern of a semiconductor device is repeatedly exposed.
2. Description of Related Art
Lithography which uses a charged particle beam such as typified by an electron beam, as compared with optical lithography, is expected to provides an advantage in terms of fineness.
However, this method is accompanied by the problem of difficulty in maintaining a high throughput. However, in LSI devices having many repeated patterns, using an EB (electron beam) mask having formed in it an extracted repeated pattern to repeatedly perform partial overall exposure, it is possible to improve the throughput.
FIG. 5(a) is a simplified drawing of an electron beam exposure apparatus that performs partial overall exposure, and FIG. 5(b) is a magnified perspective view of an aperture mask.
The electron beam that is emitted from the electron source 501 is formed into a rectangular shape by the first aperture mask 502, which is used for beam formation. As shown in FIG. 5(b), several types of patterns selected-from CAD data base of the device as partial overall patterns, are previously formed in a second aperture mask 503.
The electron beam 505 that passes through the first aperture mask 502 is deflected by a deflector above the second aperture mask 503 so as to pass through a selected pattern on the second aperture mask.
An objective lens and deflector beneath the second aperture mask 503 reduce the pattern on the second aperture mask 503 and transfer it to an appropriate position on the wafer 504.
Then, the X-Y stage (not shown in the drawing) onto which the wafer 504 is mounted is moved by one pitch, and exposure is performed on a region that is neighboring to the region which has just previously been exposed. Another method is to deflect the electron beam so as to expose a region on the wafer that is shifted by one pitch.
In the above-described method, the shot size that can be transferred at one time is limited by the electron beam optics that are used, and is at present very small compared to the size of chips.
For this reason, the pattern on a chip is divided for arrangement on the mask. However, for a DRAM or other LSI in which there are many repeated patterns, it is possible to reduce the number of patterns that are arranged on the aperture mask.
When using this partial overall exposure method, however, to prevent defects from occurring between adjacent transfer patterns and in the connection parts between transferred parts, it is necessary to eliminate mutual position shifts between shots as much as possible.
However, in partial overall exposure in which each one of multiple shots which would exceed millions of shots, is repeatedly transferred, one by one, it is difficult to completely eliminate defects that are caused by variation between shot positions.
Reasons for these variations are thought to include local position skew that is dependent upon the linearity accuracy of the D/A converter used in controlling the deflector, and suddenly occurring position skew that is caused by environmental factors such as vibration and electrical noise.
When position skew occurs because of such factors, there is a danger of shorts or bridge defects occurring at the connection part between adjacent shots.
For example, when performing partial overall exposure using the aperture mask 601 which is shown in FIG. 6(a), if each shot is made at the proper position, the transfer pattern 602, as shown in FIG. 6(b), is connected smoothly, with no breaks.
However, if there is shift between the partial overall shots, the transfer pattern will not be connected properly.
For example, if the transfer pattern 603 is shifted to the left or right with respect to the previously formed transfer pattern 604, a bridge 605 such as shown in FIG. 6(c) will occur, and if the transfer pattern 603 is shifted upward with respect to the transfer pattern 604, a short 606 will be formed, as shown in FIG. 6(d).
If the aperture mask 701 which is shown in FIG. 7(a) is used to perform partial overall exposure and the transfer pattern 703 is shifted downward with respect to the previously formed transfer pattern 702, there is a high possibility that a bridge 704 will be formed between transfer patterns, as shown in FIG. 7(b).
Because of the above-described problem, there has been a proposal in the past for the purpose of solving the problem of defects in the connection part between shots used in partial exposure.
In Japanese Unexamined Patent Publication (KOKAI) No. 57-112016 (Japanese Examined Patent Publication(KOKOKU) No. 61-45375), there is a proposed method (hereinafter referred to as the first improvement example) for use in a variable formed exposure which exposes an arbitrary rectangular pattern while moving the beam a small amount at a time.
In this method, as shown in FIG. 8, when forming a straight-line transfer pattern 801, the length of the formed beam is gradually increased in stepwise fashion for each shot to form the transfer pattern 802, and after the formed beam reaches a maximum length, this length is maintained, and a shot is performed to form the transfer pattern 803, after which at the edge at the end point to be described the formed beam length is shortened gradually each shot in stepwise fashion, so as to form the transfer pattern 804.
In the example shown in the drawing, all regions for the transfer pattern 801 are exposed four times each. It is said that according to this method, it is possible to prevent the occurrence of bulging or depressions at the joints between each shot.
In Japanese Unexamined Patent Publication (KOKAI) No. 2-71509, there is proposed a method (hereinafter referred to as the second improvement example) of reducing the defects at connection parts between shots by providing a protruding part at the edge of a pattern of an aperture mask.
Specifically, when a transfer pattern 901 is to be formed, a protruding part is provided beforehand at a pattern edge of the aperture used to perform partial overall exposure, one shot being used to perform exposure so that the corresponding protruding parts of the transfer patterns 902 are caused to overlap, thereby forming the continuous transfer pattern 901, as shown in FIG. 9.
In the past examples described with reference to FIG. 6 and FIG. 7, there were problems of bridges and shorts occurring because of position skew between shots.
In the first improvement example, which is shown in FIG. 8, there is the drawback that in this method, uses a formed beam to perform exposure, the need to perform multiple exposures as the beam shape is changed hinders the achievement of high throughput, and also the drawback that the transfer pattern that can be formed is limited to a straight-line pattern.
In the second improvement example, which is shown in FIG. 9, because of the need to form a part (such as a protrusion) that is not intrinsically part of the pattern, there is the problem of having to add a change to the pattern shape at the end of a pattern when forming the aperture mask based on CAD data.
This results in a protrusion or the like existing in the outermost part of the transfer pattern which intrinsically is not part of the pattern.
Additionally, in the second improvement example, similar to the example of the past which is shown in FIG. 7, if a vertical direction shift occurs between shots, there is an increased possibility of the occurrence of a bridge defect in the pattern between shots that are adjacent in that direction.
Therefore, an object of the present invention is to provide an exposure method which uses the partial overall exposure to suppress the occurrence of bridges between adjacent patterns, and the occurrence of opens, pattern bulging and bridges at the connection parts in a pattern, without using an aperture mask having special pattern, and without a great reduction in throughput.